1. Field of the Invention
The present invention relates to direct view displays, and more particularly to direct view liquid crystal displays employing integrated prism sheets.
2. Description of the Related Art
Backlight systems in liquid crystal displays (LCDs) tend to be inefficient, with typically only about 5-10% of the light incident on the back being transmitted through the polarizers (40%), open aperture (60%), and the color filters (35%). The operation of a conventional backlight can be understood with reference to FIG. 1.
Referring to FIG. 1, light from a triband cold cathode fluorescent light (CCFL) 10 is directed into the Acrylic light guide 12 by a reflector 15. The light only escapes from the light guide 12 if the angle of incidence at the acrylic/air interface is less than the critical angle. The light guide 12 is in the form of a wedge so that the incident angle of the light at the acrylic/air interface is gradually decreased as it propagates down the light guide by reflecting between the top and bottom surfaces 14 until it is less than the critical angle and escapes. The intensity of the light which escapes is controlled by a pattern of dots 17 printed on the bottom of the light guide 12 where the density of the dots 17 is adjusted to result in a uniform illumination of the display. Any light which exits the bottom of the guide 12 is redirected upward by a white diffuse scattering sheet 16.
The dots 17 cause diffuse scattering which results in more of the light escaping the light guide 12 in the area of the dots 17. The light typically comes out of the light guide 12 with a peak brightness at about a 70 degree angle from the normal to the display. A ridge collimating film sheet or sheets 20 (see Suzuki et. al. U.S. Pat. No. 5,600,462, for example) are used to redirect the white light so that the peak brightness is normal to the display. Additionally, the light has an appropriate angular distribution (a half brightness at 25 degrees or more off the display normal in both the vertical and horizontal directions) to provide an adequate viewing angle. With 90 degree twisted nematic (TN) mode liquid crystal 28, typically used for portable active matrix liquid crystal displays (AMLCDs), the viewing angle is usually limited by the off-normal contrast ratio and color inversion due to the liquid crystal.
For desktop monitors, a broader distribution of light is desirable: a half brightness at about 40 degrees or more off normal in both the vertical and horizontal directions. Note that it is also desirable to have a wider horizontal than vertical viewing angle. After the ridge sheet(s) 20, the light passes through a back polarizer 24, a thin film transistor (TFT) plate 26, a liquid crystal (LC) layer 28, a color filter plate 22 (in color filter displays), and a front polarizer 30 which reduces the intensity of the transmitted light to only 5-10% of that incident, as described above.
One way of improving the backlight efficiency is to use the combination of a totally internally reflecting light guide, a diffraction grating and lenticular lens to separate the Red, Green, and Blue (RGB) light by angles and then focus the individual colors through the appropriate sub-pixels, see, e.g. the commonly assigned application, to Y. Taira entitled xe2x80x9cCOLOR FLAT PANEL DISPLAY,xe2x80x9d PCT Application number JP00/00912, filed Feb. 12, 1999, designating the United States and incorporated herein by reference. This can improve the efficiency by removing the color filters (about 3xc3x97 improvement) and by focusing the light into the open aperture (about 1.3xc3x97 improvement). The general operation of the color filterless (CF-less) backlight can be understood with reference to FIG. 2.
Referring to FIG. 2, a light source 112 (CCFL) and reflector 113 direct light into an acrylic light guide 116 which has no printed dot pattern on it so that light can only escape when it""s angle of incidence is less than the critical angle. A low index coating 117 (with the refractive index, n, equal to 1.29, for example) along the bottom surface of the light guide 116 results in the light only exiting on a bottom surface 114 of the light guide 116 with a fairly narrow distribution of angles. The CCFL 112 has triband phosphors, so the light produced is mainly in three distinct Red, Green, and Blue bands. A reflective diffraction grating 115 is attached to the low index coating 117 and serves to decompose the white light into the three individual colors and redirect them upward at slightly different angles for each color. Note that a transmissive grating sheet could be used in an alternative configuration in which case no low index coating is required and a mirror sheet is placed below the light guide. A lenticular lens sheet 120 on the bottom of a back polarizer 122 of the AMLCD then focuses the angularly separated RGB light through the appropriate sub-pixels 123. With this approach, the peak intensity for the Red and Blue light is directed at an angle to the display normal. This peak off-normal Red and Blue light will lead to lateral color shifts when viewing the display. A diffuser 121 is included to improve viewing angle.
The lateral color shift problem will now illustratively be described with reference to FIG. 3. Referring to FIG. 3, the optimum focal length of a lenticular sheet 214 is fixed by the sub-pixel pitch of the display and the angular separation between the Red, Green, and Blue light. The Red 202, Green 204, and Blue 206 light are focused through the appropriate sub-pixel apertures by lenticular lens 214. The Red, Green and Blue light is focused through apertures 208 in black matrix 211, but the blue and red light are laterally shifted away from the normal (or Green light). An observer at point xe2x80x9cOxe2x80x9d would see more blue in the displayed image while an observer at point xe2x80x9cPxe2x80x9d would observe more red in the displayed image. Polarizers 212 and glass substrates 210 are provided as is known in the art. Note that in some cases, the peak red and blue intensities are not equally separated from the green intensity peak, so it may be desirable to tilt the green light slightly off normal so that the deviation of red and blue from the normal is minimized. In this case, the green light will also have a slight lateral color shift.
A color filterless backlight system has been described by van Raalte in U.S. Pat. No. 4,798,448, entitled HIGH EFFICIENCY ILLUMINATION SYSTEM FOR DISPLAY DEVICES, which used lenticular lens and a transmissive diffraction grating but did not describe any means of correcting for the lateral color shifts when viewing the display.
When microlenses are used for projection displays, as has been described by H. Hamada in xe2x80x9cOptical systems for high-luminance LC rear projectionxe2x80x9d, SID ""96 Digest, pp. 911-914, a projection lens is used in front of the AMLCD which images the black matrix plane onto the screen and hence no correction of the lateral color shift is needed. For direct view displays, in particular liquid crystal displays, a key attribute is the thickness of the display which should be as thin as possible. The use of dichroic mirrors for the angular separation of the colors, as described by Hamada, requires too much depth to be used and since the displays are viewed directly a projection lens cannot be used to fix the lateral color shift problem. An additional problem with this arrangement in projection displays is that a large projection lens is needed to collect the divergent light. A preferred method of angular color separation may include the combination of a totally internal reflecting light guide and a diffraction grating, as has been described by Taira, cited above since this is very compact and requires no additional space.
The use of combined microlenses and microprisms has been described by Nishihara in U.S. Pat. No. 5,764,319 entitled TRANSMISSIVE DISPLAY DEVICE WITH MICROLENSES AND MICROPRISMS ADJACENT COUNTER ELECTRODE, where the microlens focuses the three primary color components onto each set of three sub-pixels so that the respective color components correctly fall onto the corresponding sub-pixels. The microprisms are located between the corresponding microlenses and the sets of three sub-pixels. The microprisms convert the light ray diverging apart from the optical axis of the microlenses into a substantially parallel light ray. For projection displays, this has the advantage of allowing a smaller projection lens to be used. The microlenses and microprisms are formed on the substrate which contains the counter electrode for the liquid crystal display. For a direct view display, this arrangement is not practical since the substrate which contains the counter electrode also contains the black matrix which is directed toward the viewer to improve the display contrast by reducing reflections. The other substrate which contains the thin film transistor devices is directed toward the backlight so that the only light which is incident on the active channel of the device must first reflect off the bottom of the black matrix so that photoleakage is reduced. Another approach to fixing the lateral color shift problem for direct view displays is to use a diffusing sheet on top of the exit polarizer. Since a diffuser does not redirect the peak brightness of the light but only broadens the distribution, a very large diffusing power is necessary which results in backscattering of the light and loss of efficiency by increasing the viewing angle beyond what is needed. A diffusing sheet also has the great disadvantage that ambient light will be scattered back to the viewer and severely reduce the ambient contrast ratio. A neutral density filter can be used to reduce this backscatter, but such a solution reduces the brightness. A further disadvantage of a diffuser on or under the polarizer, as described by Miyatake et al. in xe2x80x9cDiffusive Layer for Reflective Type LCDs,xe2x80x9d International Display Workshop ""99, pp. 403-406, is that the display resolution is reduced since the light transmitted by the sub-pixel apertures expands while propagating through the top substrate to the diffuser. It is desirable to maintain the full resolution of the display.
An additional method for correcting lateral color shift is to use a transmissive hologram as has been described by Wenyon in U.S. Pat. No. 5,796,499, Hockley et al. in U.S. Pat. No. 5,046,793, or a special surface hologram formed with the method, described by Petersen in U.S. Pat. No. 5,534,386, applied in sheet form on top of the exit polarizer, all incorporated herein by reference. Additionally, the transmissive or surface hologram must not only correct the lateral color shift but it must also diffuse light to provide an adequate viewing angle for the display. One disadvantage of any color correcting sheet on the top surface is that the apparent pixel size is increased at that point by the lateral divergence of the Red and Blue light. If the thickness of the polarizers and the front and back glass are the same and if the lenticular lens focuses the light at the center of black matrix (BM) apertures, the apparent pixel size will be increased by ⅔ or more. A further issue for the transmissive hologram approach is that they are difficult and expensive to fabricate. It is not certain that a surface hologram can be fabricated which simultaneously corrects the color shift and provides a wide viewing angle with adequate light transmission.
A surface hologram embedded into a LCD has also been described by Jannson et al. in U.S. Pat. No. 5,631,754, incorporated herein by reference. The microreplication techniques and materials disclosed therein were used for features between the crossed polarizers of a TN cell and this suggests that the birefringence in the replicated structure and material is low enough not to significantly degrade the contrast ratio of the display.
In FIG. 3, if the input plane waves are all exactly parallel, the lenticular on the entrance polarizer adds a divergence of about xc2x18 degrees in the horizontal direction in the glass. A more realistic case would be to assume that the input light has a slight divergence such as xc2x12 degrees, but a larger horizontal divergence is not possible or color mixing will occur between the neighboring sub-pixels. In this case, for the configuration of FIG. 3, the overall divergence for the green light in the horizontal direction is about xc2x110 degrees in the glass, or about xc2x115 degrees in air. Even for a portable display, a wider horizontal luminance distribution is needed.
Therefore, a need exists for an apparatus which eliminates lateral color shifting, and increases horizontal viewing angle while maintaining the full resolution of a display without reducing ambient contrast ratio for direct view liquid crystal displays.
A display device provides a first optical device disposed in a light path for spatially separating angularly separated light into color components, and a pixel which receives each of the color components through a sub-pixel. Each sub-pixel controls transmitted light intensity therethrough. A black matrix is formed in operative relationship with the sub-pixels. The black matrix includes apertures for receiving the color components after the pixel. An integrated microstructured layer is disposed in the light path and receives the color components through the sub-pixels. The microstructured layer includes structured surfaces for redirecting laterally shifted color components shifted by the first optical device and optionally further diffusing all color components. The faceted surfaces redirect laterally shifted color components toward a display normal of the display device.
Another display device, in accordance with the present invention, includes a first optical device disposed in a light path for spatially separating angularly separated light into color components. A pixel receives each of the color components through a sub-pixel. Each sub-pixel controls transmitted light intensity therethrough. A black matrix is in operative relationship with the sub-pixels, the black matrix including apertures for receiving the color components after the pixel. A microstructured layer is disposed in the light path of the apertures of the black matrix, the microstructured layer including first faceted surfaces for redirecting laterally shifted color components shifted by the first optical device, the microstructured surface including a plurality of surfaces including at least one of facets, curves and angles surfaces to provide increased viewing angles for the display device.
A liquid crystal display device, in accordance with the present invention, includes a first optical device sheet disposed in a light path for spatially separating angularly separated light into color components. A first substrate has a pixel array disposed thereon, each pixel including three sub-pixels for receiving each of the color components through the sub-pixel. A second substrate is spaced apart from the first substrate by a gap, the gap being filled with liquid crystal material. The second substrate includes a microstructured layer disposed on the second substrate, the microstructured layer including faceted surfaces for redirecting laterally shifted color components shifted by the first optical device sheet, faceted surfaces for redirecting the laterally shifted light toward a display normal. The second substrate includes a common electrode formed on the microstructured layer, and a black matrix layer including apertures patterned in operative relationship with the sub-pixels for receiving the color components through the sub-pixels.
In other embodiments, the color components include red, green and blue, and the microstructured layer includes a first portion which receives a green component and angled portions inclined relative to the first portion for the red and blue components. An overcoat layer may be formed on the microstructured layer wherein the optical index of the microstructured layer is different from the optical index of the overcoat layer such that the color components are shifted in accordance with a difference between the optical index of the microstructured layer and the overcoat layer. The overcoat layer preferably forms a planar surface over the microstructured layer. The display device may include a top plate having a substrate, the microstructure layer formed on the substrate, a common electrode layer formed on the overcoat layer, and the black matrix formed over the common electrode layer. Alternately, the display device may include a top plate having a substrate, the black matrix formed on the substrate, the microstructure layer formed over the black matrix, the overcoat layer formed on the microstructure layer and a common electrode layer formed on the overcoat layer.
In still other embodiments, the device may include a light diffuser including a material having a birefringence such that a contrast ratio of 50:1 or greater for the display device is achieved. The microstructured layer may include light shaping elements for increasing viewing angle for the display device. The light shaping elements may include at least one of prisms, facets and curves. These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.